This invention relates generally to the dispensing of fluid materials onto substrates. More particularly, the invention relates to the detection of bubbles in pressurized liquid dispensing systems. Specifically, this invention relates to the detection of the presence of air bubbles in a fluid stream delivered from a nozzle of a system for dispensing adhesives, sealants, or caulks to a substrate.
The presence of an air bubble passing through a nozzle of a dispensing system may cause a void to occur in the stream of the material being dispensed, and, in turn, the bead deposited upon the substrate. If the occurrence of the air bubble is small, the effect on the resulting bead may be minimal. However, if the air bubble is large, the effect may be to produce a discontinuancy in the bead. In some applications, discontinuities of the bead may not be critical, however, in others they may be. For example, discontinuities in a bead of the adhesive/sealant applied to a windshield may not only affect its ability to act on a moisture barrier, but it also may affect the strength of the bond of the windshield to the vehicle.
Schroter U.S. Pat. No. 4,662,540 illustrates one attempt to detect the presence of air bubbles in sealants, mastics, and adhesives. In this system, a pressure transducer produces an electrical signal which corresponds to the instantaneous pressure of the fluid. This electrical signal is then differentially amplified and compared to a threshold level. In other words, U.S. Pat. No. 4,662,540 teaches the comparison of the rate of change of the pressure waveform to that of a preset reference. The rate of change of the waveform produced by the pressure sensor in excess of the threshold level is used to distinguish between bubbles and other disturbances in the pressure of the fluid which is presumed to have lower rates of changes than bubbles. However, this method is believed to have the disadvantage of not being able to distinguish between bubbles and other waveforms which may have a rate of change which resembles bubbles, but which are in fact other disturbances.
When an air bubble passes through a nozzle, a pressure disturbance occurs in the fluid upstream (in the opposite direction of the flow of the fluid) of the nozzle. The amplitude of this pressure disturbance or waveform may be large or it may be small, but its profile is distinctive. The profile begins with a transition of a negative-going extrusion of pressure, followed by a recovery portion which then overshoots the normal pressure level to produce a positive-going extrusion of pressure, before recovering to the normal operating pressure. This waveform resembles that of a pulse that is sent down a rope or a whip when it is cracked. However, other waveforms that are not bubbles may also have similar types of profiles.
Similar types of waveforms, for example, may be produced by gun turn-on and turn-off transients, pressure fluctuations of the dispensing system, electrical interference, or the passing of a solid or semi-solid chunk or piece of material (such as cured fluid) through the dispensing system. Therefore, a detector which only distinguishes disturbances based on the rate of change of the pressure waveform may result in false positive indications of bubbles.
U.S. Pat. No. 4,662,540 attempts to solve the gun turn-on and turn-off transients by disabling the system while the gun is being turned on or turned off. This, however, means the bubble detection system is blind during certain portions of the operation. It also does not distinguish between the other transient disturbances, such as the passing of a solid or semi-solid chunk or piece of material and those of bubbles.